Coated ferruginous metal and method

ABSTRACT

FERRUGINOUS METAL SUBSTRATES ARE PROTECTED WITH AN ADHERENT, CORROSION-INHIBITING, COATING WHICH IS THE RESIDUE OBTAINED BY ELEVATED TEMPERATURE CURING OF AN APPLIED ACIDIC COATING SOLUTION. THE ACIDIC COATING SOLUTION CONTAINS A HEXAVALENT-CHROMIUM-PROVIDING SUBSTANCE AN ORGANIC POLYBASIC ACID OR A SALT THEREOF, AND AN INORGANIC ACID OR SALT THEREOF. ELEVATED TEMPERATURE CURING OF SUCH SOLUTIONS APPLIED TO FERRUGINOUS SUBSTRATES PROVIDES A SURFACE COATING OF EXCELLENT CORROSION RESISTANCE.

United States Patent 3,563,811 COATED FERRUGINOUS METAL AND METHOD Jon A. de Ridder, Ashtabula, and Dick M. Warburton, Painesville, Ohio, assignors to Diamond Shamrock Corporation, Cleveland, Ohio, a corporation of Delaware N0 Drawing. Filed June 4, 1968, Ser. No. 734,242 Int. Cl. C23f .7/26

U.S. Cl. 1486.2 4 Claims ABSTRACT OF THE DISCLOSURE Ferruginous metal substrates are protected with an adherent, corrosion-inhibiting, coating which is the residue obtained by elevated temperature curing of an applied acidic coating solution. The acidic coating solution contains a hexavalent-chromium-providing substance, an organic polybasic acid or a salt thereof, and an inorganic acid or salt thereof. Elevated temperature curing of such solutions applied to ferruginous substrates provides a surface coating of excellent corrosion resistance.

BACKGROUND OF THE INVENTION A method for protection with a hexavalent-chromiumcontaining coating composition has been shown heretofore in U.S. Pat. 2,559,812. The coating solutions taught therein are acidic and are disclosed for use on zinc surfaces. Typically, such solutions are applied by dipping zinc into the coating solution or by spraying the zinc with the solution. The coating solution can be moderately heated if desired, but a room temperature bath is generally preferable. Additionally, the coating solution provides an attractive surface appearance for the zinc substrate. The disclosure of U.S. Pat. 2,559,812 is incorporated herein by reference.

SUMMARY OF THE INVENTION The teachings of U.S. Pat. 2,559,812 define surface protection at moderate to preferably room temperature operation for, solely, zinc surfaces. However, it has now been found, that if the coating solutions of such patent are applied to a ferruginous surface and, if such coated ferruginous surface is thereafter subjected to a heating step at a temperature far exceeding the maximum temperatures of 160 180 F. disclosed in such patent, e.g., an elevated temperature substantially above about 325 F., the resulting coated surface has an excellent abrasionresistant, corrosion-inhibiting coating. The corrosion resistance is moreover, greatly enhanced over that resistance obtained if the coating solution on the ferruginous substrate were merely heated at about 1602180 F.

Broadly, then, the present invention is directed to a coated ferruginous metal substrate surface comprising a coating on the surface of such substrate of between about 3200 milligrams per square foot of the residue obtained upon heating an applied acidic, corrosion-resistant, hexavalent-chromium-containing coating solution at a temperature substantially above 325 F. but below about 550 F. for a period of time sufficient to vaporize volatile substituents from the applied coating solution and deposit such residue at least substantially bonded to such ferruginous surface, wherein the coating solution is such a solution as disclosed in U.S. Pat. 2,559,812.

Additionally, this invention relates to a method for protecting ferruginous metal substrate surfaces with an adherent, corrosion resistant, hexavalent-chromiurn-containing coating solution.

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DESCRIPTION OF THE PREFERRED EMBODIMENTS The coating compositions shown in the above-mentioned U.S. patent, which compositions are also referred to herein for convenience as acidic coating compositions, are used herein to coat ferruginous metal surfaces, e.g., iron, stainless steel, or steel such as cold-rolled steel. The materials in such coating compositions include 4-18 weight percent of a hexavalent-chromium-providing substance, e.g., chromic acid, supplying at least about 4 weight percent chromic acid, and also include about 0.5-9 Weight percent of an organic polybasic acid, e.g., citric or tartaric, as well as 0.1-8 Weight percent of an inorganic acid; some salts of such acids may also be employed. Preferably, the inorganic acids used are the readily commercially available acids, and as such, are employed within the range from about 0.18%. Thus, when hydrochloric acid is used, there is preferably employed in the composition between about 0.18% of an aqueous solution containing, at atmospheric pressure, about 30 to 36 percent of hydrogen chloride.

Although the temperature of the acidic coating composition during application to the ferruginous substrate can be room temperature, it also may be at a moderate temperature of up to -180 F. Such compositions can be coated on the ferruginous surface typically by immersing the metal in a bath, or spraying the acidic coating composition on the surface, or by brushing or flowing such solution onto the surface.

After application of the acidic coating composition, the substrate is heated at a temperature substantially above about 325 F. but below about 550 F. for a period of time sufficient to vaporize volatile substituents from the applied coating solution and deposit a residue at least substantially bonded to the surface. After such heating, the substrate surface is dry to the touch and the residue sufiiciently bonded to the surface to withstand typically at least about two inch-pounds of impact without removal of coating to bare metal on the convex, i.e., reverse surface. For such impact testing a metal ram of specified weight, in pounds, with a hemispherical contact surface is allowed to drop on the coated panel from a predetermined height, in inches. Moreover, prior to such testing the panel is typically topcoated as has been more specifically described hereinbelow. Generally, especially for factory applied compositions, air drying, e.g., within the temperature range from about 65 to about 200 F. and for a time of a few minutes or less, will precede heating.

Heating at a temperature which is not substantially above about 325 F. can provide resulting coated substrates which exhibit corrosion resistance comparable or even downgraded from the corrosion resistance obtained without any heating of the applied acidic coating composition. Moreover, heating at temperatures above about 550 F. can result in some final film degradation. Preferably after the application of the acidic coating composition, the substrate is heated at a temperature between about 375 525 F. The substrate is heated for at least about 5 seconds, but, for economy, the heating is not continued for substantially more than about 10 minutes. Heating for less than about 5 seconds can be insufiicient to prepare tough, adherent coatings even for extremely elevated temperature heating. Typically the heating is accomplished by baking in a convection oven or by curing under infrared lamps.

The acidic coating compositions are applied to the substrate in an amount yielding, after curing, between about 3-200 milligrams of residue per square foot of substrate metal surface. The presence of less than about 3 milligrams per square foot of such residue may be insufficient to offer desirable enhancement in corrosion resistance and the presence of more than about 200 milligrams of coating residue per square foot can be uneconomical. Preferably, for best economy with excellent corrosion resistance, the acidic coating composition is applied in an amount to provide a coating residue of between about 30-100 milligrams per square foot.

In addition to the substituents discussed hereinabove, the coating composition may contain a non-ionic wetting agent such as alkylphenoxypolyoxyethylene ethanol, e.g., commercial nonylphenoxypolyoxyethylene ethanol, in concentrations typically up to about 3 grams per liter of the solution.

Before applying the coating composition to the substrate, it is desirable that the substrate be thoroughly cleaned. The use of a commercial alkaline cleaning composition which combines washing and mild abrasive treatments can be employed for this purpose, e.g., an aqueous trisodium phosphate-sodium hydroxide cleaning solution. In addition to cleaning, the substrate can undergo cleaning plus etching, e.g., with hydrofluoric acid etching agent. To accomplish the substrate etching and instead of applying only the acidic coating solution, a mixture can be applied which incorporates an etching agent in with such solution. In lieu of a clean metal surface, an additional suitable surface for applying the coating composition is one wherein the metal substrate has been treated to exhibit a loose, powdery residue which is retained on the substrate for subsequent application of the acidic coating composition. Such residues can promote adhesion for later applied paints.

After baking the acidic coating composition on the ferruginous substrate it can be topcoated with any suitable paint, i.e., a paint, primer, including electrocoating primers, enamel, varnish, or lacquer. Such paints can contain pigment in a hinder or can be unpigmented, e.g., generally cellulose lacquers, rosin varnishes, and oleoresinous varnishes, as for example tung oil varnish. The paints can be solvent reduced or they can be water reduced, e.g., latex or water-soluble resins, including modified or soluble alkyds, or the paints can have reactive solvents such as in the polyesters or polyurethanes. Additional suitable paints which can be used include oil paints, including phenolic resin paints, solvent-reduced alkyds, epoxys, acrylics, vinyl, including polyvinyl butyral and oil-wax-type coatings such as linseed oil-parafiin wax paints. The paints can be applied as mill finishes.

The following example shows a way in which the invention has been practiced but should not be construed as limiting the invention.

PREPARATION OF TEST PANELS Steel test panels (typically 4" x 12", and being cold rolled, low carbon steel panels, or plain, mild steel panels, or hot-dipped galvanized steel panels) are prepared for coating application by immersing in water which has incorporated therein typically about 30 grams of cleaning solution per liter of water. The cleaning solution is prepared from phosphoric acid, potassium hydroxide, and a 4 wetting agent and the cleaning bath is maintained at a temperature of 160-180 F. Additionally, before cleaning, the mild steel and galvanized steel panels are vapor degreased with perchlorethylene and subsequently heated to 350 F. for 10 minutes.

APPLICATION OF COATING COMPOSITION Unless otherwise indicated in the example, the coating composition is applied by dipping the test panel into such composition, removing and draining excess composition from the panel, air drying at room temperature until the coating is dry to the touch prior to any curing. Panels which are then selected for curing are typically placed in a convection oven for a time up to about 6 minutes thereby achieving substrate temperatures as shown in the example.

CORROSION RESISTANCE TEST (ASTM Bl1764) Corrosion resistance of coated panels is measured by means of the standard salt spray (fog) test for paints and varnishes, ASTM B-l1764. In this test, panels are placed in a chamber kept at constant temperature where they are exposed to a fine spray (fog) of a 5% salt solution for specified periods of time, rinsed in water and dried. The extent of corrosion and film removal on the test panels can then be measured in inches of coating failure away from scribe lines as explained in greater detail hereinafter in the example.

PAINT FILM The paint film (topcoat) referred to in the example is a commercial white alkyd enamel topcoat typically applied by dip-coating panels into the enamel. This paint is prepared from a modified alkyd resin based upon a system of partially polymerized phthalic acid and glycerine. The paint contains weight percent solids and has a viscosity of 50 seconds as measured on a No. 4 Ford cup at 70 F. After coating panels with the enamel, the coating is cured by baking in a convection oven for 20 minutes at a temperature of 320325 F.

EXAMPLE Panels are prepared in the manner described hereinabove and coating compositions are applied to these panels by the method disclosed hereinbefore. As shown in the table below, the compositions employed include two acidic coating compositions as well as a comparative chromic acid/succinic acid/succinimide/phosphoric acid composition. All applied coating compositions are cured at temperatures reported in the table below. Additionally, the table shows the results of the corrosion resistance test (salt spray) and each reported figure is an average from at least two tested panels.

The metal substrates employed are shown in the table, and all panels have been topcoated with the alkyd enamel topcoat, and the applied topcoat cured, as abovedescribed. The figures presented in the table for the corrosion resistance test, e.g., indicate the average inches of coating failure away from scribe lines which have been cut through to the base metal, in an X configuration on the panel surface, prior to subjecting the panels to the test.

TABLE Concen- Salt tration, p y, Ingredlents g ll Surface Cure temperature 168 hour CrO; Mild steel 435-435 F e52 su ccinip a cid 22 --do Room temperature 2%; g f }Galvanized steel 435435 F 9%; scrgauufinfl. 435435" F 1 0 mm c roma CCF succin-ic acid 30 All are low carbon steel 9%2 Zinc n1trate 10 Room temperature" 8%:

22 53235333 111: 20 Mild steel 4 5-4 5 F 39a HaPOr acid 2 4. 2

The first composition reported in the table has been taught heretofore for use as a corrosion-inhibiting coating for Zinc substrates, wherein such use is unaccompanied by elevated temperature curing. This teaching is supported by the results shown above, that is, the curing at elevated temperature of such composition on zinc (the galvanized steel substrate), provides very poor corrosion protection for the zinc.

However, as will be readily seen from the results in the above table, the elevated temperature curing, i.e., the 425 435 F. curing, for the acidic coating compositions applied over ferruginous substrates provides coated surfaces achieving excellent, enhanced corrosion protection. Thus for the steel surface and the first reported compo sition, an enhancement in coating integrity of more than five times greater is achieved by elevated temperature curing rather than simply room temperature curing. Furthermore, for the second composition reported, and again on a steel surface, the enhancement in coating integrity for elevated temperature curing, as opposed to either room temperature of 180 F. moderate temperature curing, is even more pronounced. This superior corrosion protection over ferruginous substrates and with elevated temperature curing, is further highlighted upon comparison of these results with the lessened protection afforded by the comparative chromic acid/succinic acid/ succinimide/phosphoric acid composition, which in itself shows desirable corrosion resistance.

In a further test, the comparative coating composition, i.e., the last composition reported in the table, is also applied to, and cured on, mild steel panels according to the methods described hereinbefore and the coating composition has the same ingredients with the exception that only 10 grams per liter (g./l.) of succinimide are employed in such compositions. Comparative panels, some cured at room temperature, and some cured at the elevated temperature of 425-435 F., but all topcoated with the standard alkyd topcoat, are all subjected for 168 hours to the corrosion resistance test d scribed hereinbefore. The panels cured at the elevated temperature show an average coating failure away from scribe lines of of an inch. However, the panels which are merely air dried show an average coating failure under the same conditions of only 6 of an inch.

Thus, the results shown in the table for the first-reported composition demonstrate that elevated temperature curing for the acidic composition on zinc substrates can be deleterious. But, these last results with the comparative composition containing 10 g./l. of succinimide demonstrate that, even with steel substrates, protected with an acidic, hexavalent-chromium-containing coating solution, elevated temperature curing for such a solution on such a substrate may be deleterious.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

We claim:

1. The method of protecting a ferruginous metal substrate surface with an adherent residue from an acidic, corrosion resistant, hexavalent-chromium-containing coating solution wherein the protected metal surface exhibits enhanced corrosion resistance, which method comprises:

applying to the ferruginous surface said acidic coating solution; and

heating said substrate at a temperature substantially above about 325 F. but below about 550 F. for a period of time sufficient to vaporize volatile substituents from the applied coating solution and deposit an acidic coating solution residue at least substantially bonded to said surface; wherein said coating solution comprises:

(a) 48 weight percent of a substance selected from the group consisting of chromic acid, sodium and potassium salts thereof, and mixtures of same, which substance supplies at least about 4 weight percent chromic acid;

(b) 0.5-9 weight percent of a substance selected from the group consisting of tartaric acid, succinic acid, citric acid, sodium and potassium salts thereof, and mixtures of same;

(c) 0.1-8 weight percent of an acidic compound selected from the group consisting of hydrochloric, sulfuric, nitric, and phosphoric acids, and sodium and potassium salts thereof, and mixtures of same;

wherein the salts in said groups (a), (b), and (c) are present in said solution in anionic equivalent amounts of the amount of acid specified.

2. The method of claim 1 wherein said acidic coating solution for application to said substrate is maintained at a temperature not substantially above about F. and after application said substrate is heated at a temperature maintained within the range from about 375 F. to about 525 F. and for a time of at least 5 seconds.

3. The method of claim 2 wherein volatile substituents from said coating solution are at least in part evaporated from the applied solution prior to said heating and said residue is present on said surface after heating in an amount of between about 3-200 milligrams per square foot.

4. A coated ferruginous metal substrate comprising a coating on the surface of said substrate of between about 3-200 milligrams per square foot of the residue obtained upon heating an applied corrosion resistant, acidic, hexavalent-chromium-containing coating solution at a substrate temperature substantially above about 325 F. but below about 550 F. for a period of time sufficient to vaporize volatile substituents from the applied coating so lution and deposit said residue at least substantially bonded to said surface, wherein said acidic coating solution comprises:

(a) 4-18 weight percent of a substance selected from the group consisting of chromic acid, sodium and potassium salts thereof, and mixtures of same, which substance supplies at least about 4 weight percent chromic acid;

(b) 0.5-9 weight percent of a substance selected from the group consisting of tartaric acid, succinic acid, citric acid, sodium and potassium salts thereof, and mixtures of same;

(c) 0.1-8 weight percent of an acidic compound selected from the group consisting of hydrochloric, sulfuric, nitric, and phosphoric acids, and sodium and potassium salts thereof, and mixtures of same;

wherein the salts in said groups (a), (b), and (c) are present in said solution in anionic equivalent amounts of the amount of acid specified.

References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner c. B. WESTON, Assistant Examiner US. Cl. X.R. 1486.l6, 6.21 

